The invention relates to a circuit arrangement for serial data transmission and more particularly to such an arrangement having a plurality of subscriber stations interconnected over a bus, which stations can communicate with a further station over a data transmission network.
A circuit arrangement of this kind has already been diclosed in the German OS No. 3433150. The known circuit arrangement contains a drive circuit with two opto-couplers connected in series with one another. A drive module for the opto-couplers comprises a control circuit which, on the occurrence of a static fault signal, via a timer or time circuit blocks the connection to the opto-couplers of a single station. This prevents the data transmission line from becoming blocked to all other subscribers in the event of a static fault signal at one station.
As, subscriber circuits are connected in parallel to the bus, a control circuit's liability to break down increases in proportion to the number of subscriber circuits connected to the bus. Therefore in circuit arrangements comprising a comparatively large number of subscriber circuit's it can occur that, when the availability of the circuit arrangement is subject to particuarly high usage requirements, these requirements cannot be fulfilled by the known measures.
Therefore the aim of the invention is to design a circuit arrangement of the type referred to in the introduction, in such manner that it exhibits the highest possible level of availability even in the case of a comparatively large number of subscriber stations. In particular, the facility is to be provided with means for locating out-of-order subscriber stations.
In accordance with the invention, a circuit arrangement for serial data transmission is designed with a coupling device interconnecting the bus of the subscriber stations with a data transmission network, such coupling device incorporating a time circuit having a longer time constant than the time circuits assigned to the individual subscriber stations.
These measures result in the advantage that the circuit arrangement for serial data transmission is effectively protected both in respect of static fault signals of subscriber circuits and also in respect of control circuit faults blocking the data transmission network.
German Published Application 3436441 discloses a data communications facility for serial data transmission that comprises a data network having a tree structure. The stations are arranged in a plurality of network levels. Signal collectors, that collect and forward signals, are situated at the network nodes. Since the signal collectors do not produce a connection between a plurality of bus systems, the data transmission network cannot be blocked by static error signals. However, microprocessors or the like are required in each of the network nodes.
In the circuit arrangement of the present invention, static error signals can be identified because the data to be transmitted are supplied to a timer circuit that responds only to given static signals, i.e. to signals that are active over a long time span, but does not respond to the transmission of signals having status changes that follow one another relatively fast.
In an improvement of the invention, in the event that the subscriber circuits transmit pulse sequences of a predetermined maximum duration, the control circuit is designed so as to respond to the overshooting of the predetermined, maximum duration. Here it can be expedient that the pulse sequence be converted into a continuous signal, in particular with the assistance of a retriggerable monoflop, and that the continuous signal be monitored by means of the control circuit. These measures are particularly advantageous if the bus comprises a control bus in addition to a data bus, so that the control circuit need only analyze the logic level at a control output of the subscriber circuit on the control bus, in order to monitor the predetermined, maximum duration of the pulse sequence.
Other measures achieve the advantage that once a static fault signal has occurred, it triggers a permanent, enforced disconnection. Then the subscriber circuit or group of subscriber circuits in question no longer responds. The absence of a response can serve as a criterion for the location of out-of-order subscriber stations. In the improbable event that a control circuit, assigned to a coupling device, is itself out-of-order, the group of subscriber circuits which includes the subscriber circuit whose control circuit is defective can be identified at the central station.
The desired enforced clearance of the bus and the data transmission network is expediently effected by logic means.
Display and alarm devices are provided, which serve to further simplify the fault locating. These measures are particularly advantageous if the groups of subscriber circuits which are connected to a common data bus are arranged at one and the same location, such as in a spatially adjacent array.
The measure for resetting, in the event of a temporary interruption, achieves the advantage that the self-holding of the time circuit is terminated by interrupting the power to the device which contains the self-holding time circuit, such as disconnecting it from a power supply receptacle. If the subscriber circuit and the control circuit, and the coupling device and the control circuit, are in each case accommodated in one and the same device which must anyhow be disconnected from its power source receptacle for the repairs required in the event of a fault, the desired termination of the self-holding takes place automatically in this way.
Logic means is provided for the self-holding of the time circuit.
Another aspect of the invention provides means which are simple to implement, in the event of an impermissibly long seizure of the bus, the subscriber circuit in question is unable to influence the data bus and the coupling device in question is unable to influence the data transmission network.
These and other objects and advantages will become clear from a review of the following description and the accompanying drawings.